1. Field of the Invention
Wounds resulting from burns, severe abrasions, skin ulcers, skin transplants, and the like often affect relatively large areas of skin and are particularly vulnerable to infection. To protect such wounds, particular dressings or coverings have been developed which are often referred to as "synthetic skins" or "artificial skins." Such artificial skins should have a number of properties in order to effectively protect the wound while healing or prior to subsequent treatment such as homographt. First, they must have an optimal water permeability which avoids fluid accumulation under the covering while simultaneously preventing desiccation of the wound. This is a particular problem in view of the high rate of water loss in wounds where the epidermal layer has been partially or completely lost, typically in the range from about 3400 to 5200 g/m.sup.2 /24 hr. See, Lamke (1977) Burns 3:159. This compares to an average water loss through the epidermal layer on the order of 240 g/m.sup.2 /24 hrs.
The material used as an artificial skin should also act as a barrier to prevent microbial invasion, be flexible so that it will conform to the wound as the body is in motion, inhibit scar formation and be capable of adhering to the wound while allowing intentional removal without damage to the underlying tissue. The material must also be biocompatible, i.e., be non-toxic, non-antigenic and non-irritating.
Heretofore, various polymeric membranes have been developed for use as wound coverings. One such wound covering which is commercially available is Op-Site.RTM., manufactured by T. J. Smith and Nephew Ltd., Kingston-Upon-Hull, United Kingdom. Op-Site.RTM. is a thin polyurethane film having a pressure-sensitive layer on one surface. Although useful, the Op-Site.RTM. dressing suffers from certain disadvantages. It displays limited water permeability, typically on the order of 500 g/m.sup.2 /24 hr. Such low permeability often leads to fluid accumulation under the dressing. Also, the application of an adhesive on the wounded area can sometimes irritate or inflame the wound.
Therefore, it would be desirable to provide a synthetic wound covering having the desirable characteristics set forth above.
2. Description of the Prior Art
U.S. Pat. No. 3,645,835 to Hodgson describes polymeric wound coverings comprising a layer of a synthetic polymer, e.g., polyurethane, having a pressure-sensitive adhesive on one surface. This patent appears to cover the Op-Site.RTM. material described above. Other commercially available polymeric bandaging materials include Epigard, a laminate formed from a polyurethane foam and a polypropylene film which is available from Parke Davis Co., Detroit, Mich. and Lyofoam, a polyurethane foam heated on one surface to form a film. U.S. Pat. No. 3,800,792 describes a polyurethane collagen composite used as a bandaging material. The grafting of hydrophilic monomers onto hydrophobic polymers is known. See, e.g., Chapiro, "Radiation Chemistry of Polymeric Systems," Interscience, Academic Press, London 1962. Radiation-induced grafting of acrylic monomers onto the surface of a polyurethane film is described in Ratner et al. (1978) J. Appl. Polym. Sci. 22:643-664, and Hoffman and Ratner (1979) Rad. Phys. Chem. 14:831-840. Jansen and Ellinghorst (1979) J. Polym. Symp. 66:465-473 disclose a pre-swelling technique to form an interpenetrating network of hydroxyethyl methacrylate and a polyether polyurethane. Cerium (Ce.sup.+4) salt was used as an initiator for grafting of polyacrylamide onto the surface of a polyurethane film. See, Halpern et al. Annual Report PH-43-66-1124-6, PB-230, National Institutes of Health (1974). A paper describing clinical results obtained using the wound covering of the present invention was presented at the VII International Congress of Plastic and Reconstructive Surgery, Montreal, Canada, June 26-July 1, 1983, Transactions pp. 26-27. A similar paper was presented in San Francisco at the VI International Congress on Burns, Aug. 29-Sept. 1, 1982.